1. Field of the Invention
The present invention comprises an apparatus and method for the controlled release of a substance through a membrane-based formulation system. In particular, the invention relates to an apparatus and methods for controlling the rate of release of a substance through a membrane-based formulation system at substantially constant rates over long periods of time.
2. Background Art
Formulation systems for controlled release of chemical substance such as odors, fragrances, perfumes and insect semiochemicals are of considerable importance. They are widely used in agriculture, households, restaurants, hospitals and other daily life as well as commercial applications. For example, most chemical insect traps used in agriculture and daily life contain a lure. The lure is a formulation system for releasing a chemical substance to attract the targeted insects to the trap. Formulation systems designed for applications requiring large numbers of devices preferably should be economically constructed. The release of the chemical substance preferably should be persistent and consistent over an extended time period.
Considerable effort has been directed to the development of formulation systems for use in release of various chemical substances. More than 30 U.S. and foreign patents have been resulted. U.S. Pat. No. 4,614,299 issued to Van Louvern et al. summarized various apparatus and methods resulting from this effort and disclosed by inventors over the world, and itself disclosed a membrane-based formulation system which utilizes microporous polymers. Formulation systems are also described in numerous publications, reviews, book chapters and books. "Insect Suppression with Controlled Release Pheromone Systems" edited by Kydonieus, Beroza, and Zweig, CRC Press, Inc. Boca Raton, Fla. (1982), Vol. 1 and 2; and "Insect Pheromone Technology: Chemistry and Applications" edited by Leonhardt and Beroza, American Chemical Society Symposium Series 190, American Chemical Society, Washington, D.C. (1982) are examples of books addressing the subject.
Despite the fact that many compounds can be formulated with existing formulation systems there remains a need to increase the duration over which that the active substance is released, to provide a constant release rate over time, and to develop formulations with the ability to control the release of highly volatile chemicals. One particular problem associated with existing formulation systems such as rubber matrices, laminates, and polyvinyl polymers is that they do not provide constant release of highly volatile chemicals over the time periods needed for bioassays and field tests of many insect semiochemicals. As discussed in the paper "Prediction of Release Ratios of Multicomponent Pheromone from Rubber Septa", Heath, et al., J. Chem. Ecol., 12(12), 2133-2143 (1986), use of rubber septa as formulation devices is limited to compounds that have a vapor pressure of less than 1.0 Pa. Release rates of compounds with vapor pressures of approximately 1.0 Pa that have been loaded on rubber septa will typically decrease to one half of their original release rates (half-life) in approximately 14 days.
Methods to formulate highly volatile compounds using capillaries filled to variable heights have been described, for example, in the paper "Identification of Floral Compounds of Night-Blooming Jessamine Attractive to Cabbage Looper Moths", Heath, et al., Environ. Entomol., 21(4), 854-859 (1992). This formulation method provides systems for use in laboratory bioassays but is more difficult to use in the field because the capillary must be maintained in a vertical position and because the lure must be protected from adverse weather.
Membrane-based formulation systems, such as that disclosed in U.S. Pat. No. 4,614,299 issued to Van Louvern et al., have also been developed over the years. Of the different formulation systems available, membrane-based formulation systems appear to be the most cost effective. Because membrane-based formulation systems follow zero-order kinetics, they provide a constant emission of substance over time with a linear decrease in the amount of material remaining in the formulation. These systems use various materials such as particulates or pulverized fillers such as clay, limestone, silicates, polymers, etc., and/or extending agents such as high molecular weight polymers and various nonvolatile chemicals added to the active compound tested time periods. Currently, the commercially available formulation of trimedlure ("TML") that is most widely used is based on incorporation of 2 ml of trimedlure in a thermoset plastic plug manufactured by Magnet lure, AgriSense, Fresno, Calif. As disclosed in the paper "A Controlled Release Formulation of Trimedlure Isomer C and Its Attractiveness to the Mediterranean Fruit Fly (Diptera: Tephritidae)", Heath, et al., J. Econ. Entomol., 83(3), 819-822 (1990), this formulation releases isomer C at approximately 200 .mu.g/hr and has a half-life of approximately 21 days, which is an improvement over other more involved systems such as laminates. However, in the recent experience of the inventor and that of United States Department of Agriculture--Animal and Plant Health Inspection Service (USDA-APHIS) personnel the formation of crystals on the surface of the plugs have been observed. These crystals were identified as isomer C based on gas chromatographic data and mass spectroscopy. These crystals often dislodge from the surface of the plug and potentially provide an attractive source away from the trap.
Finding a better formulation system becomes more urgent because of the great threat posed by various fruit flies to the well-being of agriculture around the world. It is estimated that California, Texas and Florida alone deploy an excess of 5,000 lures in the field per year to detect pest fruit flies. These lures typically must be replaced every two weeks because no active substance is released thereafter. Approximately 20 countries also use formulations of trimedlure to monitor for the Mediterranean fruit fly. These efforts, however, have been thwarted or at least weakened by lack of a suitable formulation. The inventor's recent efforts testing various fruit fly attractants, including the four component blend of hexanol, ethyl hexanoate, ethyl octanoate, and cineole, previously identified as food attractants from chapote for the Mexican fruit fly, experienced the same problems. In previous tests using materials that were formulated using rubber septa or formulations developed by Scentry.RTM. were unable to demonstrate biological activity as disclosed in the paper "A Novel Attractant for Mexican Fruit Fly, Anastrepha Ludens, from Fermented Host Fruit," Robacker, et al., J. Chem. Ecol., 16, 2799-2815(1990). This, combined with the crystallization problem associated with the current membrane-based formulation system, has prompted further investigation of alternative substance release mechanism and systems.